1. Field of the Invention
The invention relates to an image reading apparatus and an image reading system for converting the image of an original into an electric signal.
2. Related Art
An image reading apparatus of, e.g., flat bed type includes, as known well, an original table constructed of a transparent board, a light source for irradiating an original, and a line sensor having a multiplicity of photoelectric conversion elements. Such image reading apparatus irradiates a beam of light onto the original placed on the original table, detects reflected light or transmitted light while moving the line sensor, and converts density changes in the original into an electric signal.
However, since an original, from which only small reflected light or transmitted light is obtained, gives small input light intensity to the line sensor, the input light amount to the sensor (input light intensity.times.light storing time) becomes close to the noise level, which in turn imposes the problem of impaired image quality.
To overcome this problem, an image reading apparatus has been proposed in, e.g., Unexamined Japanese Patent Publication No. Hei. 4-335760. This image reading apparatus is provided as increasing the level of a light amount signal from the line sensor by increasing the input light amount to the line sensor with the light storing time per line increased in accordance with the density of an original if the density of the original is high as well as the input light intensity to the sensor is small, and as converting the light amount signal whose level has been increased into an image signal using a gamma function.
According to this image reading apparatus, although the lightness of the original is increased, the original reading speed is reduced in proportion to the increase in the light storing time, which is a problem. In addition, the user has to alter the operation panel settings of the image reading apparatus in accordance with a change in density of an original every time the original is changed, which imposes another problem of cumbersome operation.
To overcome these problems, proposed is a method of increasing the level of a light amount signal by increasing the irradiation intensity of a light source with the original reading speed kept at a conventional level.
However, this method sacrifices the life of the light source to increase the irradiation intensity; i.e., the service life of the light source is inevitably shortened.